The present invention relates generally to fluorescent illuminating devices, and, more particularly, to an inductive-resistive fluorescent apparatus and method.
Fluorescent lamps are well known in the prior art. There are three basic types of such lamps. These are the preheat lamp, the instant-start lamp, and the rapid-start lamp. In each type of lamp, a glass tube is provided which has a coating of phosphor powder on the inside of the tube. Electrodes are disposed at opposite ends of the tube. The tube is filled with an inert gas, such as argon, and a small amount of mercury. Electrons emitted from the electrodes strike mercury atoms contained within the tube, causing the mercury atoms to emit ultraviolet radiation. The ultraviolet radiation is absorbed by the phosphor powder, which in turn emits visible light via a fluorescent process.
The differences between the three lamp types generally relate to the manner in which the lamp is initially started. Referring now to FIG. 1, in a preheat lamp circuit, designated generally as 10, a starter bulb 12 is included. Preheat lamp 14 includes first and second electrodes 16 and 18, each of which has two terminals 20. During initial start-up of the preheat lamp, starter bulb 12, which acts as a switch, is closed, thus shorting electrodes 16 and 18 together. Current therefore passes through electrode 16 and then through electrode 18. This current serves to preheat the electrodes, making them more susceptible to emission of electrons. After a suitable time period has elapsed, during which the electrodes 16 and 18 have warmed up, the starter bulb 12 opens, and thus, anelectric potential is now applied between electrodes 16 and 18, resulting in electron emission between the two electrodes, with subsequent operation of the lamp.
A relatively high voltage is applied initially for starting purposes. A lower voltage is used during normal operation. A reactance is placed in series with the lamp to absorb any difference between the applied and operating voltages, in order to prevent damage to the lamp. The reactance, suitable transformers, capacitors, and other required starting and operating components are contained within a device known as a ballast (designated generally as 22). Ballasts are relatively large, heavy and expensive, with inherent efficiency limitations and difficulties in operating at low temperatures. The components within ballasts are typically potted with a thermally conductive, electrically insulating compound, in an effort to dissipate the heat generated by the components of the ballast. Difficulties in heat dissipation are yet another disadvantage of conventional ballasts.
Referring now to FIG. 2, an instant-start lamp circuit, designated generally as 24, is shown. Instant-start lamp 26 includes first and second electrodes 28 and 30. Electrodes 28 and 30 each only have a single terminal designated as 32. In operation of the instant-start lamp, no preheating of the electrodes is required. Rather, an extremely high starting voltage is typically applied in order to induce current flow without preheating of the electrodes. The high starting voltage is supplied by a special instant-start ballast, designated generally as 34. Instant-start type ballasts suffer from similar disadvantages to those of the preheat type. Further, because of the danger of the high starting voltage from the instant-start ballast 34, a special disconnect lamp holder 36 must be employed in order to disconnect the ballast when the lamp 26 is not properly secured in position.
Referring now to FIG. 3, a rapid-start lamp circuit, designated generally as 38, is shown. Rapid start lamp 40 includes first and second electrodes 42 and 44, each of which has two terminals 46, similar to the preheat lamp 14, discussed above. The rapid-start ballast, designated generally as 48, contains transformer windings which continuously provide the appropriate voltage and current for heating of the electrodes 42 and 44. Rapid heating of electrodes 42 and 44 permits relatively fast development of an arc from electrode 42 to electrode 44 using only the applied voltage from the secondary windings present in ballast 48. The rapid start ballast 48 permits relatively quick lamp starting, with smaller ballasts than those required for instant-start lamps, and without flicker which may be associated with preheat lamps. Further, no starter bulb is required. However, ballast 48 is still relatively large, heavy, inefficient, and unsuitable to low ambient-temperature operation. Dimming and flashing of rapid-start lamps are possible, albeit with the use of special ballasts and circuits.
It will be appreciated that operation of the prior art lamps described above is dependant on heating of the electrodes and/or application of a high voltage between the electrodes in order to start the operation of the lamp. This necessitates the use of ballasts and associated control circuitry, having the undesirable attributes discussed above. Recently, there has been interest in employing other physical phenomena to enable efficient starting and operation of fluorescent lamps. For example, EPO Publication Number 0 593 312 A2 discloses a fluorescent light source illuminated by means of an RF (radio frequency) electromagnetic field. However, the device of the '312 publication still suffers from numerous disadvantages, including the complex circuitry required to generate the RF field and the potential for RF interference.
In the parent International Application No. PCT/US97/18650, a ballast-free drive circuit is disclosed which, in one embodiment, employs a direct current (DC) or pulsed DC source (see FIG. 25). It has been found, however, that operating a fluorescent lamp with a DC or pulsed DC source can lead to mercury migration in the lamp and an associated reduction of light output over time. This mercury migration problem may, therefore, substantially shorten the usable life of the fluorescent lamp.
Through experimentation, it was additionally observed that the fluorescent lamp drive circuit disclosed in the parent International Application exhibited unreliable starting of the fluorescent lamp, particularly when used with certain types of fluorescent lamps (e.g., T8 lamps). This starting problem was found to be related, at least in part, to an insufficient voltage being generated across the output capacitors in the drive circuit. In such instances, the capacitors were not always fully charged to an appropriate voltage level necessary to form the arc in the fluorescent medium.
There is, therefore, a need in the prior art for an inductive-resistive fluorescent apparatus which permits simple, economical and reliable starting and operation of fluorescent lamps with low-cost, light weight, low-volume components which are capable of efficiently operating the lamp, even at relatively low ambient temperatures, which afford efficient heat dissipation and which are capable of operating at ordinary household AC frequencies. It is desirable to adapt such an inductive-resistive fluorescent apparatus to substantially eliminate mercury migration in the fluorescent lamp. It is additionally desirable to provide a fluorescent apparatus having the flexibility for enhanced features, including the ability to remotely control the fluorescent apparatus via a proportional industrial controller (PIC) or similar building controller. Furthermore, it is desirable to adapt such an inductive-resistive apparatus to direct "plug-in" replacement of incandescent bulbs.